Method for applying a bonding layer

ABSTRACT

A method for applying a bonding layer that is comprised of a basic layer and a protective layer on a substrate with the following method steps: application of an oxidizable basic material as a basic layer on a bonding side of the substrate, at least partial covering of the basic layer with a protective material that is at least partially dissolvable in the basic material as a protective layer. In addition, the invention relates to a corresponding substrate.

FIELD OF INVENTION

The invention relates to a method for applying a bonding layer and to asubstrate provided with a bonding layer.

BACKGROUND OF INVENTION

In the state of the art, countless methods to connect various materialsto one another exist. In the semiconductor industry, in recent years,primarily the bonding technology to connect two substrates temporarilyor permanently to one another has gained acceptance. Very often, thebonding process takes place between semiconductor(s) and/or metalstructures on the substrate. The best-known metal bonding technology ofrecent times is copper bonding. Substrates are the carriers forfunctional assemblies such as microchips, memory chips, or MEMSassemblies. In recent years, increasingly attempts were made to producea connection between assemblies arranged on various substrates in orderto avoid a wire-bonding process between the assemblies that islabor-intensive, costly, and susceptible to flaws. In addition, thedirect bonding variant has the enormous advantage of elevated assemblydensity. The assemblies must no longer be positioned beside one anotherand connected via wires, but rather are stacked on one another andconnected vertically to one another by various technologies. In mostcases, the vertical connections are produced by contact points. Thecontact points of different substrates must be identical to one anotherand are oriented to one another before the actual bonding process.

Another little-used process is aluminum bonding. In this process,aluminized points on the surface of a substrate are to be bonded with amaterial lying on a second substrate. In this case, this can be aluminumor a suitable, different material. One drawback of aluminum is itsextreme oxygen affinity. Even with copper, the oxygen affinity is high,so that copper oxides must be regularly removed before a bondingprocess. With aluminum, the oxygen affinity is higher by a multiple.Aluminum still forms relatively thick, passivating aluminum oxidelayers, which are difficult to remove. In contrast to copper, aluminumis therefore little used for bonding connections, since at this time,because of the very stable oxide layers, no reliable bonding result canbe achieved at a reasonable cost. Nevertheless, aluminum is widely usedin the semiconductor area to produce metal connections on the chipsurface in the lateral direction. Here, aluminum is distinguished inthat it has significantly slower diffusion behavior in silicon than, forexample, copper or gold. Metal diffused into silicon would impair thecharacteristics of transistors or make the latter completelyunfunctional. Based on this advantageous diffusion behavior, paired withlow costs and relatively good electrical conductivity, aluminum has beenestablished over many years as the material that is mainly used forproducing electrical connections laterally on the semiconductor chips.Recently, in chips of the newest generation, aluminum is increasinglybeing replaced by copper because of its better electrical conductivity;however, aluminum still enjoys great importance, primarily in theproduction of chips on 200 mm substrates with somewhat older productiontechnology. It is specifically these production surrounding areas/plantsthat in recent times have found enhanced use for the production of MEMS(micro-electro-mechanical systems) components. The production of theseMEMS components in turn frequently requires bonding processes, so thatthe need for a reliable aluminum bonding process increases. Outside ofthe semiconductor industry, aluminum is also a structural material thatis in demand, since it is light, inexpensive, and primarily hardenable.In the semiconductor industry, based on the above-mentioned reasons, ithas been attempted for some time to develop processes that make aluminumusable as structural material and in particular material for bondingconnections.

The greatest problem when using oxygen-affine materials such as copperand aluminum is the avoidance of oxidation on bonding surfaces and/orthe complete removal of oxide from bonding surfaces before a bondingprocess. Extremely oxygen-affine materials such as aluminum produce,moreover, oxides that are strong and difficult to reduce. Plants foroxide removal are expensive, labor-intensive and under certaincircumstances dangerous (toxic substances).

SUMMARY OF INVENTION

The object of this invention is therefore to indicate a method as wellas a substrate provided with a bonding layer, with which an oxidizablematerial (such as in particular aluminum) can be used for bonding.

This object is achieved with the features of the independent claim(s).Advantageous further developments of the invention are indicated in thesubclaims. All combinations of at least two of the features indicated inthe specification, the claims and/or the figures also fall within thescope of the invention. In the indicated ranges of values, values thatlie within the above-mentioned limits are also to be disclosed asboundary values and can be claimed in any combination.

It is an essential idea of this invention to provide a bonding layer,comprised of a basic layer and a protective layer, in particular as adiffusion pair, on the substrate, whereby a basic material of the basiclayer can be oxidized, while a protective material of the protectivelayer can be oxidized at least less easily.

The invention therefore in particular is a matter of a process in whichthe oxidation of oxygen-affine materials such as in particular aluminum(preferred) or copper is prevented from the start. The protection ofoxygen-affine basic material is achieved according to the invention inparticular by the deposition of a protective material, which covers thebasic material at least partially, in particular primarily, andpreferably completely.

The objective breakdown of the elements relative to their oxygenaffinity can be most simply defined by the electrochemical voltagesequence. Oxygen-affine elements such as lithium are extremely basematerials, are easily oxidized, and therefore act as reducing agents,easily release electrons, and therefore have an extremely negativestandard electrode potential. However, elements with a low oxygenaffinity are referred to as noble, since the latter can be easilyreduced and therefore act as oxidizing agents, accommodate electrons,and have an extremely positive standard electrode potential. As basicmaterial, in particular a material with a standard electrode potentialis used that is less than 2.00 V, preferably less than 1.00 V, morepreferably less than 0.0 V, most preferably less than −1.0 V, withutmost preference less than −2.0 V, and even more preferably less than−3.0 V. Copper has a standard electrode potential of approximately 0.16V, aluminum of approximately −1.66 V. The noblest metal is gold with astandard electrode potential of approximately 1.69 V (for the firstoxidation stage).

In an especially preferred variant, the basic material and theprotective material are located as targets, separated from one another,in a coating chamber and are applied in succession under vacuum, so thatno contact of the basic material with an oxygen-containing atmosphere isproduced.

An embodiment of the invention is comprised in leaving the protectivematerial on the basic material during the bonding process and, becauseof its chemical-physical properties, in at least partially, inparticular predominantly, and preferably completely dissolving theprotective material in the basic material during the bonding process.The selection of the basic material-protective material combination iscarried out in such a way that the latter allows a solid dissolutionprocess. The protective material is preferably more readily soluble inthe basic material than vice versa.

The protective material is dissolved in particular under specificprocess conditions in the basic material. The basic material thereforehas a boundary solubility for the protective material and/or the basicmaterial can be mixed at least partially, in particular predominantly,and preferably completely with the basic material. In the case of anexisting boundary solubility of the protective material in the basicmaterial, the boundary solubility at room temperature is in particulargreat enough to keep a specific amount of the protective materialdissolved. In this way, the protective material according to theinvention can be applied as an extremely thin layer in order to avoidlocal concentration peaks during the diffusion process of the protectivematerial into the basic material that can lead to an (undesired)precipitate.

Another aspect of this invention that is advantageous according to theinvention is comprised in preventing a contact of the oxygen-affinebasic material with an oxygen-containing or oxygen-rich atmosphere, inparticular by at least predominant covering of the surfaces of the basiclayer, not covered by the substrate, with the protective layer.

The protective material itself is preferably a solid, in particular atleast at room temperature. The latter is therefore not liquid and allowsthe transport of the protected basic material through anoxygen-containing atmosphere.

In an advantageous embodiment of the invention, the protective materialis selected in such a way that the latter is less oxygen-affine than thebasic material, or any oxide that is formed on the protective materialcan be removed with simpler means than would be the case in an oxideformed on the basic material. Oxide-forming material for the protectivelayer is advantageously selected in such a way that in addition to thesimple removal of the oxide, new oxide is only slowly formed again afterthe oxide removal. In particular, less than 0.3 nm of oxide, preferablyless than 0.1 nm of oxide, is formed on time lapses of at least 2minutes, preferably at least 5 minutes, more preferably at least 10minutes, and most preferably at least 15 minutes.

An incorporation of any oxide, formed on the protective layer, in thebasic material is in particular at least predominantly preventedaccording to the invention. To this end, the oxide of the protectivematerial is removed in particular immediately before a bonding process.In the case of a smaller amount of formed oxide, a breaking of the oxideduring the later desired bonding process and a direct incorporation intothe boundary layer are also conceivable. Protective materials with oneor more of the properties cited below are preferably used:

-   -   A low oxygen affinity, in particular defined by a standard        electrode potential of more than 0 V, preferably more than 1.00        V, more preferably more than 2.00 V, preferably less than the        oxygen affinity of the basic material,    -   A high solubility in the basic material, in particular more than        10-5 mol %, preferably more than 10-3 mol %, more preferably        more than 1 mol %, most preferably more than 10 mol %, and with        utmost preference more than 40 mol %,    -   The properties of the basic material are not negatively        influenced, and therefore in the case of the desired high        conductivity do not impair the conductivity and in the case of        the desired high level of strength do not reduce the strength,    -   Airtight relative to the atmosphere,    -   Economical,    -   High level of availability,    -   Slightly toxic, in particular non-toxic, and/or    -   Good bonding properties.

Accordingly, the invention relates in particular to a method forcovering, in particular coating, a first material tending towardoxidation, the basic material, in particular a metal or a semiconductor,with a second material, the protective material. The protective materialis in particular dissolved at least partially, in particularpredominantly, and preferably completely in another process step by adissolution process of the basic material and/or forms precipitatespartially, in particular predominantly, and preferably completely, in aquite special, expanded embodiment. The embodiment that is preferredaccording to the invention comprises a complete dissolution of theprotective material into the basic material, whereby in this case, noprecipitates are produced. The object of the protective layer formedfrom the protective material consists in particular in the prevention ofthe oxidation of the basic material. The protective material can itselfoxidize upon contact with an oxygen-containing atmosphere and isoptionally removed from oxide, before the dissolution process accordingto the invention begins in the basic material. In a quite specialembodiment, the removal of this oxide on the protective layer isperformed in a unit in which a renewed oxidation of the protective layeris prevented for design reasons on the way to the bonder. For example,the use of an oxide removal module and a bonding chamber in acorresponding vacuum cluster, which separates the module from thesurrounding, oxygen-containing atmosphere, would be conceivable. Suchcluster systems are well known to one skilled in the art.

The preferred target of the process according to the invention primarilyincludes protecting oxygen-affine basic materials, in particularaluminum but also copper, which are to be bonded in an additionalprocess step, up to the actual bonding step before an oxidation. Theremoval of any oxides of the protective material, which preferably has amuch lower oxygen affinity than the basic material to be protected, isconsiderably simpler, faster, and primarily more reliable, so that theprocess can be accelerated.

According to a preferred embodiment, the protective material, inparticular after a removal of oxides of the protective material, is thenbonded with another substrate that is formed in particular according tothe invention. Preferably, the protective material is at leastpartially, in particular predominantly, and preferably completelydissolved during the bonding process in the basic material, so that inthe ideal case, no contact occurs between the basic material and theoxygen-rich atmosphere until the bonding takes place.

The method according to the invention is therefore primarily suitablefor temporarily protecting with the protective material oxygen-affinebasic materials, in particular aluminum or copper, directly after thecoating on a substrate.

To the extent the basic material is not full-surface but rather isapplied on the substrate in particular in a structured manner and/oronly in partial areas (for example copper contacts or aluminumboundaries, which are to be part of a hermetically-sealed space of aMEMS component), the protective material is applied in the form of afilm that is as uniformly thick as possible, and in particular issealed. According to the invention, primarily the ratio of the (inparticular middle) layer thickness of the basic material to the layerthickness of the protective material is important. In addition, thechemical, physical and/or metallurgical behavior of the basic materialwith the protective material according to the invention plays a role.

The basic material is preferably a solid solvent. In particular, thebasic material can be a multi-phase-multi-component system. In the caseof a multi-phase material, all considerations for the dissolutionprocess are valid at least for a phase, in the ideal case for allphases. As basic material, an individual chemical element is preferablyselected, in particular a metal, a semi-metal, or a non-metal, inparticular silicon, gallium, aluminum, nickel, titanium or copper. Thesemetals are the materials that are most often used in the semiconductorindustry for the production of conductive compounds, contact orstructural building blocks (for example MEMS assemblies).

In order to disclose the process according to the invention as simply aspossible, the process according to the invention is described by way ofexample, but not in a limiting manner, with regard to the basic materialaluminum. Aluminum is especially suitable according to the invention,since it is a highly-available, economical structural material.

The protective material can in particular also be amulti-phase-multi-component system, but it is preferably a simplechemical element that preferably occurs only in one phase. In this case,it is preferably a metal, a semi-metal, an alkali metal or analkaline-earth metal. The use of non-metallic elements such as carbonwould also be conceivable according to the invention as long as thechemical-physical properties correspond between the non-metal with basicmaterial in terms of the process according to the invention.

The following materials are considered according to the invention inprinciple as basic material and/or protective material. In this case,the processes according to the invention require that the protectivematerial be at least partially, in particular predominantly, andpreferably completely, dissolvable in the basic material.

-   -   Metals, in particular        -   Cu, Ag, Au, Al, Fe, Ni, Co, Pt, W, Cr, Pb, Ti, Te, Sn, Zn,            Ga    -   Alkali metals, in particular        -   Li, Na, K, Rb, Cs    -   Alkaline-earth metals, in particular        -   Mg, Ca, Sr, Ba    -   Alloys    -   Semiconductors, provided in particular with corresponding doping        -   Element semiconductors, in particular            -   Si, Ge, Se, Te, B, Sn        -   Compound semiconductors, in particular            -   GaAs, GaN, InP, InxGal-xN, InSb, InAs, GaSb, AlN, InN,                GaP, BeTe, ZnO, CuInGaSe₂, ZnS, ZnSe, ZnTe, CdS, CdSe,                CdTe, Hg(1-x)Cd(x)Te, BeSe, HgS, AlxGa1-xAs, GaS, GaSe,                GaTe, InS, InSe, InTe, CuInSe₂, CuInS₂, CuInGaS₂, SiC,                SiGe.

For removal of oxide layers of the protective material, preferably thefollowing processes are suitable:

-   -   Chemical oxide removal, in particular by a        -   Gaseous reducing agent and/or        -   Liquid reducing agent    -   Physical oxide removal, in particular with plasma    -   Ion-Assisted Chemical Etching, in particular        -   Fast Ion Bombardment (FAB, sputtering)        -   Grinding, and/or        -   Polishing.

For deposition and therefore synthesis of the basic material and/or theprotective layer, the following processes are suitable:

-   -   Physical Vapor Deposition (English: Physical Vapor Deposition,        PVD)    -   Chemical Vapor Deposition (English: Chemical Vapor Deposition,        CVD)    -   Galvanic method    -   Sol-gel method

The systems of basic layer and protective layer according to theinvention are designed as layer systems and in particular represent asystem that is not located in thermodynamic equilibrium. As a result, ata temperature that is elevated relative to room temperature, aninterdiffusion, preferably at least predominantly and in particularexclusively, brings about a diffusion of the protective material intothe basic material between basic material and protective material.

The systems according to the invention are designed in particular as adiffusion pair. The described phase diagram as well as any phase diagramof the above-mentioned material combinations represent equilibriumstates of several phases at various temperatures and concentrations. Theconclusions that can be drawn from equilibrium diagrams such as phasediagrams regarding kinetic processes such as diffusion are very limited.In principle, equilibrium diagrams do not allow any assessment onkinetic processes. The phase diagrams are therefore used exclusively tomake an assessment as to whether the protective material in generalcould dissolve at a specific temperature in the basic material. To theextent that local concentration build-ups occur during the diffusionprocess of the protective material into the basic material, which exceedthe solubility of the protective material in the basic material and thuslead to a possible separation, phase formation or the like, this isignored in the description below. Hereinafter, in principle, theassumption is that the diffusion of the protective material into thebasic material is carried out quickly, so that at a given temperature,at no time anywhere in the basic material is the maximum solubility ofprotective material exceeded. This is achieved in particular accordingto the invention in that the greater the solubility of the protectivematerial in the basic material at a given temperature, the faster theprotective material diffuses into the basic material and/or the smallerthe transfer of protective material into the basic material in theprotective-material-basic material interface.

The depicted and described phase diagrams were determined by metallurgy.Many components have a very low solubility in the second components ineach case, so that the solubility limit is hardly recognizable based onthe depiction.

In order to be able to better describe the ideas according to theinvention, the idea according to the invention of several systems thatare as simple as possible is described. As basic material, thetechnically very important and to date material that is very difficultto bond, aluminum, is used. Therefore, basic material after thedeposition on the bonding side of the substrate is a one-componentone-phase system.

As protective materials for the protective layer, four importantmaterials are presented below by way of example, and said materials havethe necessary properties according to the invention, namely germanium,gallium, zinc and magnesium. Therefore, this protective material afterthe deposition is also a one-component-one-phase system. Theabove-mentioned material combinations are preferred according to theinvention, whereby reference is made to the examples described belowwith the above-mentioned advantages relative to the individual materialcombination.

The basic material-protective material system of the bonding layer isaccordingly a layer system, which is converted preferably during thebonding process by a dissolution process into a two-component-one-phasesystem. In this case, a mixed crystal that comprised of basic materialand protective material is preferably produced. The bonding processitself takes place either in an inert gas atmosphere, but morepreferably in a vacuum.

In an advantageous embodiment of this invention, precipitates aredesired, in particular by at least one heat treatment being carried outafter the (successful) bonding process, in order to produce atwo-component-two-phase system by at least partial, in particularpredominant, and preferably complete, precipitation of the protectivematerial out of the basic material.

Below, based on more advantageous embodiments, the invention isexplained, whereby the embodiments in each case can be consideredindependent invention aspects per se, which aspects are to be disclosedas separate inventions and are to be claimable, in particular incombination with the above general disclosure.

First Embodiment

A first system, to which the idea according to the invention can beapplied, is the aluminum-germanium system, Al—Ge for short. The binaryAl—Ge system is a purely eutectic system with a partial boundarysolubility for germanium in aluminum and a disappearing low boundarysolubility of aluminum in germanium. As basic material, aluminum istherefore selected.

In order to protect the aluminum against oxidation, it is immediatelycovered with a germanium layer as a protective layer after thesuccessful deposition on the substrate. Germanium is therefore theprotective material according to the invention.

The germanium layer is in particular less than 10 μm, preferably lessthan 1 μm, more preferably less than 100 nm, most preferably less than10 nm, and with utmost preference less than 1 nm.

The deposition is carried out at the lowest possible temperatures, inorder to prevent or at least to suppress a partial or even completediffusion of the germanium at elevated temperature into the aluminum.

In the case of the deposition of the germanium, the temperature of thealuminum is less than 600° C., preferably less than 500° C., morepreferably less than 400° C., most preferably less than 300° C., withutmost preference less than 200° C., and even more preferably less than100° C. In special cases, the aluminum can even be actively cooled inorder to further drop the temperature. By the lowest possibletemperature, a germanium that is deposited on the aluminum isimmediately hindered in its thermal movement and preferably remains onthe surface, and therefore does not diffuse into the aluminum.

In addition, the diffusion of the germanium into the aluminum ishampered by the especially low solubility of the germanium in aluminumat low temperatures. Starting from this time, germanium serves as aprotective material for the aluminum. If the system is exposed to anoxygen-containing atmosphere, the germanium oxidizes at leastpredominantly and preferably completely, and thus aluminum protects thelatter against oxidation by virtue of the fact that the aluminum issealed relative to the atmosphere.

In this connection, it is to be considered that the standard electrodepotential of germanium is approximately 0.12 V and that of aluminum isapproximately −1.66 V. Germanium is therefore nobler than aluminum andis consequently not able to protect aluminum chemically as a sacrificeanode.

Accordingly, the germanium layer is tightly applied in order to build upa physical barrier between the aluminum and the atmosphere.

In order to implement a bond between aluminum and a desired secondmaterial, first any germanium oxides formed are removed from thegermanium. The removal of the germanium oxides is carried out byphysical and/or chemical means. Sputtering-off of oxides, wet-chemicalremoval by reducing acids, or reduction by hydrogen or other gaseousreducing agents is conceivable. After the removal of germanium oxides,bringing the pure germanium surface into contact with the surface to bebonded, in particular a substrate that is structured analogously inparticular according to the invention, is carried out as quickly aspossible.

The bonding process is carried out at a bonding temperature that iselevated relative to room temperature. In this case, the bondingtemperature is in particular greater than 25° C., preferably greaterthan 100° C., more preferably greater than 200° C., most preferablygreater than 300° C., with utmost preference greater than 400° C., andeven more preferably around 426° C. According to the phase diagram,aluminum at approximately 426° C. has the greatest solubility forgermanium of approximately 2.5 mol %. According to the invention, in apreferred embodiment, the forming of a liquid, eutectic phase in theboundary area is prevented by the preferred bonding temperature beingbelow the eutectic temperature, in particular between 400° C.-420° C. Inthis temperature range, the solubility of the germanium in the aluminumis always still high enough to justify dissolution of the germanium intoaluminum. According to the invention, the bonding temperature during thebonding process is kept constant at this temperature up to the at leastpredominant, preferably complete, dissolution of the germanium intoaluminum.

The time required for the dissolution can be calculated by thedissolution of the one-dimensional diffusion equation knowing thediffusion constants of germanium into aluminum. Nevertheless, it may benecessary and useful to maintain the temperature for a shorter or longertime. The length of time for dissolving the germanium in aluminum isset, according to the invention, in particular at greater than 1 minute,preferably greater than 10 minutes, more preferably greater than 30minutes, most preferably greater than 1 hour, with utmost preferencegreater than 2 hours, and even more preferably greater than 5 hours.

During the dissolution process, the pressure on the substrates that areto be bonded with one another is preferably maintained or increased. Thepressure that acts on the bonding layer is in particular greater than 1Pa, preferably greater than 100 Pa, more preferably greater than 10,000Pa, most preferably greater than 1 MPa, with utmost preference greaterthan 10 MPa, and even more preferably greater than 100 MPa. The forcesthat are used, in particular relative to standard wafers, are greaterthan 10 N, preferably greater than 100 N, most preferably greater than1,000 N, with utmost preference greater than 10,000 N, and even morepreferably greater than 100,000 N.

During the dissolution process, the germanium preferably dissolves inall of the aluminum, Because of the fact that the amount of germanium tobe dissolved is very low at the same time that the amount of dissolvingaluminum is very large, the total concentration of the germanium inaluminum is very small. The total concentration of the germanium inaluminum is in particular less than 1 mol %, preferably less than 10⁻³mol %, preferably less than 10⁻⁵ mol %, and most preferably less than10⁻⁷ mol %. The germanium is dissolved preferably not only exclusivelyin the region of the aluminum that is near the surface but ratherdiffuses as deep as possible into the aluminum, preferably so deep thatafter a specific time, an even distribution of germanium in aluminum hasbeen achieved.

In a first procedure according to the invention, it is provided thatduring the cooling process, it does not result in exceeding the boundarysolubility of the germanium in aluminum, so that germanium alwaysremains completely dissolved in the aluminum. As a result, theprecipitation of germanium in the aluminum matrix is prevented in theentire temperature range. This is carried out according to the inventionby a ratio of aluminum layer thickness to germanium layer thicknessaccording to the invention being selected and the diffusion processrunning for a specific amount of time until germanium is distributedinto aluminum in particular predominantly, preferably completely, andprimarily over the entire available space. The ratio between thegermanium layer thickness and aluminum layer thickness is in this caseless than 1, preferably less than 10⁻³, more preferably less than 10⁻⁵,most preferably less than 10⁻⁷, with utmost preference less than 10⁻⁹,and most preferably less than 10⁻¹¹.

In an alternative procedure according to the invention, the germaniumlayer thickness is set in such a way that at higher temperatures, an inparticular at least predominant, preferably complete, dissolution of thegermanium is carried out, hut a supersaturated mixed crystal is producedduring cooling that leads to germanium precipitates. These germaniumprecipitates can positively influence the strength properties of thealuminum. They preferably result in an increase in strength of thealuminum, in particular in combination with an additional heattreatment.

Second Embodiment

A second system, to which the idea according to the invention can beapplied, is aluminum-gallium, Al—Ga for short. The binaryaluminum-gallium system is a purely eutectic system with very strongdegradation. The eutectic concentration is very close to theconcentration of pure gallium.

The boundary solubility of the gallium in aluminum is extraordinarilyhigh and reaches its maximum of approximately 7.5-8.0 mol % attemperatures around 125° C. The boundary solubility of aluminum ingallium is minute, however.

As basic material, the aluminum according to the invention is thereforeselected. In order to protect the aluminum against oxidation, it isimmediately covered with a gallium layer after the successfuldeposition. The gallium layer is formed in particular at less than 10μm, preferably less than 1 μm, more preferably less than 100 nm, mostpreferably less than 10 nm, and with utmost preference less than 1 nm.

The deposition is carried out at the lowest possible temperatures inorder to prevent or at least to suppress a partial or even completediffusion of gallium at elevated temperature into aluminum. Gallium hasa very low melting point of approximately 30° C. In order to prevent thegallium layer applied on aluminum from liquefying, a temperature ofbelow 30° C. is set. According to the invention, it would beconceivable, however, that at higher temperatures, the gallium remainsin liquid form on the aluminum, without hampering the handling of theentire wafer. The reason may be primarily the extremely small amount ofdeposited gallium, which has a high enough surface tension and a highenough adhesion to aluminum to continue to exist as a liquid metal film.

In the second embodiment, it is provided according to the invention thatthe gallium diffuses at moderate temperatures into aluminum. Therefore,the subsequent bonding process is performed as shortly as possible afterthe basic layer is covered with the protective layer.

In the case of the deposition of the gallium, the temperature of thealuminum is less than 300° C., preferably less than 200° C., morepreferably less than 100° C., most preferably less than 50° C., withutmost preference less than 30° C., and even more preferably less than0° C. In special cases, the aluminum can even be actively cooled byfurther dropping the temperature. If the system is exposed to anoxygen-containing atmosphere, the gallium preferably oxidizes and thusprotects the aluminum.

In this connection, it is to be considered that the standard electrodepotential of gallium is approximately −0.53 V and that of aluminum isapproximately −1.66 V. Gallium is therefore nobler than aluminum and isconsequently not able to protect aluminum chemically as a sacrificeanode. Accordingly, the gallium layer is tightly applied in order tobuild up a physical barrier between the aluminum and the atmosphere.

The boundary solubility of gallium in aluminum is also still extremelyhigh at room temperature, if not even just below at the maximum of thealready mentioned 7.5-8.0 mol %. The boundary solubility of the galliumin aluminum decreases again only below room temperature. A precipitationof dissolved gallium in aluminum can therefore be avoided in thisembodiment of the invention.

By the especially high solubility of gallium in aluminum, in particularalso even at room temperature, the material gallium is especiallysuitable in this respect to be dissolved in aluminum.

According to the invention, the process parameters are set so that theconcentration of gallium in aluminum at any time is less than theboundary solubility, since otherwise a two-phase system can be producedfrom an aluminum mixed crystal with gallium and a liquid phase. Thiswould have the effect that the bonding can no longer be implementedsince the liquid phase also still exists at room temperature.

In contrast, specifically the low melting point and the possibility toliquefy at extremely low temperatures are an optimal requirement for asubsequent bonding process. By means of the most minor temperatureincreases, gallium is liquefied on the surface of the aluminum and thusas a liquid phase matches the contours of the two surfaces that are tobe connected to one another. Although it is the actual idea of theinvention according to the invention to dissolve gallium in aluminum,the capability of liquefaction at low temperatures for supporting thebonding process before the actual dissolution process is disclosed inthis respect as an independent aspect of the invention.

In order to implement a bonding between aluminum and a desired secondmaterial, first any gallium oxides formed are removed from the gallium.Just like aluminum, gallium coats itself with a thick oxide layer and isthus passivated. Gallium forms a gallium hydroxide layer with water. Theremoval of gallium oxides is carried out by physical and/or chemicalmeans.

Sputtering-off of oxides, wet-chemical removal by reducing acids and/orliquors, or reduction by hydrogen or other gaseous reducing agents isconceivable. After the removal of gallium oxides, bringing the puregallium surface into contact with the surface to be bonded, inparticular a substrate that is structured analogously in particularaccording to the invention, is carried out as quickly as possible.

The bonding process is carried out at a temperature that is elevatedrelative to room temperature. In this case, the bonding temperature isin particular greater than 25° C., preferably greater than 100° C., morepreferably greater than 200° C., most preferably greater than 300° C.,with utmost preference greater than 400° C., and even more preferablyaround 426° C. According to the phase diagram, aluminum has the greatestsolubility for germanium of approximately 8 mol % between 77° C. and177° C.

Preventing the forming of a liquid, eutectic phase in the boundary areais very difficult in the case of an Al—Ga diffusion pair. Sincedissolving gallium in aluminum is preferred according to the inventionand since diffusion is to take place as quickly as possible, a briefexistence of a liquid phase according to the invention is acceptable.According to the invention, the bonding temperature during the bondingprocess is kept constant at this temperature up to the at leastpredominant, preferably complete, dissolution of gallium in aluminum.

The required time can be calculated by solving the one-dimensionaldiffusion equation knowing the diffusion constants of gallium inaluminum. Nevertheless, it may be necessary and useful to maintain thetemperature for a shorter or longer time. The length of time fordissolving gallium in aluminum is in this case greater than 1 minute,preferably greater than 10 minutes, more preferably greater than 30minutes, most preferably greater than 1 hour, with utmost preferencegreater than 2 hours, and even more preferably greater than 5 hours.

During the dissolution process, the pressure is preferably maintained oreven increased on the substrates that are to be bonded with one another.The prevailing pressure is in particular greater than 1 Pa, preferablygreater than 100 Pa, more preferably greater than 10,000 Pa, mostpreferably greater than 1 MPa, with utmost preference greater than 10MPa, and even more preferably greater than 100 MPa. The forces that areused, in particular relative to standard wafers, are greater than 10 N,preferably greater than 100 N, most preferably greater than 1,000 N,with utmost preference greater than 10,000 N, and even more preferablygreater than 100,000 N.

During the dissolution process, gallium preferably dissolves in all ofthe aluminum. Because of the fact that the amount of gallium to bedissolved is very small but the amount of aluminum to be dissolved isvery large, the total concentration of gallium in aluminum is verysmall. The total concentration of gallium in aluminum is in particularless than 10 mol %, preferably less than 5 mol %, preferably less than 1mol %, and most preferably less than 10⁻³ mol %. Gallium is preferablydissolved not only exclusively in the region of the aluminum that isnear the surface but rather diffuses as deep as possible into thealuminum, preferably so deep that after a specific time, an evendistribution of gallium in aluminum has been achieved.

In a procedure according to the invention, it is now ensured that duringthe cooling process, the boundary solubility of gallium in aluminum isnever exceeded, so that gallium always remains completely dissolved inaluminum. As a result, the precipitation of gallium in the aluminummatrix is prevented in the entire temperature range. In the Al—Gasystem, this is technically very simple to bring about since the changein the boundary solubility of gallium in aluminum is marginal in thetemperature range between approximately 130° C. and room temperature,i.e., it does not change particularly much. As a result, during thecooling process, there is virtually no danger that a (significant)precipitation of gallium in aluminum occurs. The ratio between thegallium layer thickness and the aluminum layer thickness is in this caseless than 1, preferably less than 10⁻³, more preferably less than 10⁻⁵,most preferably less than 10⁻⁷, with utmost preference less than 10⁻⁹,and even more preferably less than 10⁻¹¹.

Third Embodiment

A third system, to which the idea according to the invention can beapplied, is the aluminum-zinc system, Al—Zn for short. The binaryaluminum-zinc system is a binary system that has a zinc-rich eutecticand a zinc-rich eutectoid. For the ideas according to the invention, inparticular the boundary solubilities of the system partners areimportant. According to the Al—Zn phase diagram, aluminum has a boundarysolubility for zinc, and zinc has a boundary solubility, albeit a smallone, for aluminum. Since aluminum is preferably used as basic materialand zinc is preferably used as a protective layer, only thealuminum-rich side of the phase diagram is important.

In order to protect aluminum against oxidation, it is immediatelycovered with a zinc layer as a protective layer after the successfuldeposition on the substrate. Zinc is thus the protective material forprotection of the basic layer against oxidation.

The zinc layer is in particular less than 10 μm, preferably less than 1μm, more preferably less than 100 nm, most preferably less than 10 nm,and with utmost preference less than 1 nm.

The deposition is carried out at the lowest possible temperatures inorder to prevent or at least to suppress a partial or even completediffusion of zinc at elevated temperature into aluminum.

In the case of the deposition of zinc, the temperature of aluminum isless than 600° C., preferably less than 500° C., more preferably lessthan 400° C., most preferably less than 300° C., with utmost preferenceless than 200° C., and even more preferably less than 100° C. In specialcases, the aluminum can even be actively cooled in order to further dropthe temperature. By the lowest possible temperature, the zinc that isdeposited on the aluminum is immediately impeded in its thermal movementand preferably remains on the surface, and it therefore does not diffuseinto aluminum.

In addition, by the especially low solubility of zinc in aluminum at lowtemperatures, the diffusion of zinc into aluminum is hampered. From thistime, zinc serves as protective material for aluminum. If the system isexposed to an oxygen-containing atmosphere, the zinc at leastpredominantly, preferably completely, oxidizes, and the latter thusprotects the aluminum against oxidation, in particular by the aluminumbeing sealed relative to the atmosphere.

In this connection, it is to be considered that the standard electrodepotential of zinc is approximately −0.76 V, and that of aluminum isapproximately −1.66 V. Zinc is therefore nobler than aluminum and isconsequently not able to protect aluminum chemically as a sacrificeanode. Accordingly, the zinc layer is tightly applied in order to buildup a physical barrier between aluminum and the surrounding area, inparticular the atmosphere.

In order to implement a bond between aluminum and a desired secondmaterial, first any zinc oxides formed are removed from the zinc. Theremoval of the zinc oxides is carried out in particular by physicaland/or chemical means. Sputtering-off of oxides, wet-chemical removal byreducing acids, or reduction by hydrogen or other gaseous reducingagents, in particular carbon monoxide, is conceivable according to theinvention.

After the removal of zinc oxides, bringing the pure zinc surface intocontact with the surface to be bonded is carried out as quickly aspossible. The bonding process is carried out at an elevated temperaturerelative to room temperature. In this case, the bonding temperature isgreater than 25° C., preferably greater than 100° C., more preferablygreater than 200° C., most preferably greater than 300° C., with utmostpreference greater than 400° C., and even more preferably around 380° C.According to the phase diagram, of between approximately 350° C. andapproximately 380° C., zinc has an extremely large area with a highsolubility for zinc in aluminum.

According to the invention, the amount of deposited zinc is set low sothat after the complete and primarily uniform distribution of zinc inaluminum, no concentrations are set in a range above 1 mol % of zinc,much less 50-60 mol % of zinc. The large solubility range is well suitedin this respect to avoid any local concentration peaks without exceedingthe concentration range of a mixed crystal that is desired according tothe invention. By a correspondingly long heat treatment, anyconcentration peak of the zinc in aluminum is again reduced by a uniformdistribution of zinc in aluminum so that the final end concentration ofthe zinc in aluminum, reached before the cooling process, preferablylies below the boundary solubility of the zinc in aluminum at roomtemperature.

According to the invention, preferably during the bonding process, thesystem is therefore maintained within this temperature range for thelength of time that is required in order to dissolve all of the zinc inaluminum. Nevertheless, even at approximately 280° C., the boundarysolubility of the zinc in aluminum is high enough to implement theprocess according to the invention. The required time can be calculatedby solving the one-dimensional diffusion equation knowing the diffusionconstants of zinc in aluminum. Nevertheless, it may be useful accordingto the invention to maintain the temperature for a shorter or longertime.

The length of time for dissolving zinc in aluminum is set in particularat greater than 1 minute, preferably greater than 10 minutes, morepreferably greater than 30 minutes, most preferably greater than 1 hour,with utmost preference greater than 2 hours, and even more preferablygreater than 5 hours.

During the dissolution process, the pressure is preferably maintained orincreased on the substrates that are to be bonded with one another. Theprevailing pressure is in particular greater than 1 Pa, preferablygreater than 100 Pa, more preferably greater than 10,000 Pa, mostpreferably greater than 1 MPa, with utmost preference greater than 10MPa, and even more preferably greater than 100 MPa. The forces that areused, in particular relative to standard wafers, are greater than 10 N,preferably greater than 100 N, most preferably greater than 1,000 N,with utmost preference greater than 10,000 N, and even more preferablygreater than 100,000 N.

During the dissolution process, the zinc is preferably dissolved in allof the aluminum. Because of the fact that the amount of zinc to bedissolved is very small but the amount of the dissolving aluminum isvery large, the total concentration of zinc in aluminum is very small.The zinc is preferably dissolved not only exclusively in the region ofthe aluminum that is near to the surface, but rather diffuses as deep aspossible into the aluminum, preferably so deep that after a specifictime, an even distribution of zinc in aluminum has been achieved.

In a first procedure according to the invention, it is now ensured thatduring the cooling process, the boundary solubility of zinc in aluminumis not exceeded, so that zinc always remains completely dissolved inaluminum. As a result, the precipitation of zinc in the aluminum matrixis prevented in the entire temperature range according to the invention.This is carried out according to the invention by a ratio of aluminumlayer thickness to zinc layer thickness according to the invention beingselected and the diffusion process being run for a specific time untilthe zinc is distributed completely, in particular in the entireavailable space, in aluminum. The ratio between the zinc layer thicknessand aluminum layer thickness in this case is less than 1, preferablyless than 10⁻³, more preferably less than 10⁻⁵, most preferably lessthan 10⁻⁷, with utmost preference less than 10⁻⁹, and even morepreferably less than 10⁻¹¹.

According to another procedure according to the invention, the zinclayer thickness is set in such away that at higher temperatures, adissolution of the zinc into aluminum that is in particular at leastpredominant and preferably complete, is carried out, but during cooling,a supersaturated mixed crystal is produced that results in zincprecipitates. These zinc precipitates can positively influence thestrength properties of aluminum. They preferably result in an increasein strength of the aluminum, in particular in combination with a heattreatment.

Fourth Embodiment

A fourth system, to which the idea according to the invention can beapplied, is the aluminum-magnesium system, Al—Mg for short. The binaryAl—Mg system is a binary system that consists of two eutectics withboundary solubility for magnesium in aluminum as well as a boundarysolubility for aluminum in magnesium. As basic material, aluminum ispreferably selected.

In order to protect the aluminum against oxidation, it is immediatelycovered with a magnesium layer after the successful deposition of themagnesium material. Magnesium is a very reactive alkaline-earth metal,primarily in its pure form.

The magnesium layer is in particular less than 10 μm, preferably lessthan 1 μm, more preferably less than 100 nm, most preferably less than10 nm, and with utmost preference less than 1 nm.

The deposition is carried out at the lowest possible temperatures inorder to prevent or at least to suppress a partial or even completediffusion of the magnesium at elevated temperature in the aluminum.

In the case of the deposition of magnesium, the temperature of aluminumis less than 600° C., preferably less than 500° C., more preferably lessthan 400° C., most preferably less than 300° C., with utmost preferenceless than 200° C., and even more preferably less than 100° C. In specialcases, the aluminum can even be actively cooled in order to further dropthe temperature. Because of the lowest possible temperature, a magnesiumthat is deposited on the aluminum is immediately hindered in its thermalmovement and preferably remains on the surface, i.e., does not diffuseinto the aluminum.

In addition, the diffusion of magnesium into aluminum is hampered by theespecially low solubility of magnesium in aluminum at low temperatures.Starting from this time, magnesium serves as protective material foraluminum. If the system is exposed to an oxygen-containing atmosphere,the magnesium preferably oxidizes and thus protects the aluminum.

In this connection, it is to be considered that the standard electrodepotential of magnesium is approximately −2.36 and that of aluminum isapproximately −1.66 V. Magnesium is therefore a more base material thanaluminum and is consequently able to protect aluminum chemically as asacrifice anode. It would perhaps even be conceivable that the depositedmagnesium is used directly as a reducing agent for aluminum oxide thatis already at least partially formed or not completely removed. Thereduction process can be performed by a separate, additional heattreatment step and preferably reduces the magnesium-covered aluminumoxide to form pure aluminum with the formation of magnesium oxide.Hereinafter, in particular an independent aspect of the invention isfound that, in combination with any additional method featuresdisclosed, is to be valid and is to be claimable.

In order to implement a bond between aluminum and a desired secondmaterial, first any magnesium oxides formed must be removed frommagnesium. The removal of the magnesium oxides is carried out byphysical and/or chemical means. Sputtering-off of oxides, wet-chemicalremoval by reducing acids, or reduction by hydrogen or other gaseousreducing agents is conceivable according to the invention. It is to beconsidered in this connection that magnesium oxides should be fairlystable structures that can be removed completely by wet-chemical meansonly with great difficulty; thus, physical methods according to theinvention are rather suitable.

After the removal of magnesium oxides, bringing the pure magnesiumsurface into contact with the surface to be bonded is carried out asquickly as possible. The bonding process is carried out at an elevatedtemperature. In this case, the bonding temperature is greater than 25°C., preferably greater than 100° C., more preferably greater than 200°C., most preferably greater than 300° C., with utmost preference greaterthan 400° C., and even more preferably around 426° C.

According to the phase diagram, aluminum at approximately 452° C. hasthe greatest solubility for magnesium of approximately 16 mol %.However, if it is desired to prevent a liquid phase from forming in theboundary area and only to provide that the solid magnesium is dissolvedin solid aluminum, the preferred bonding temperature is below theabove-mentioned 452° C. In this temperature range, the solubility of themagnesium in aluminum is always still high enough to produce anoticeable dissolution of the magnesium in aluminum. According to theinvention, the temperature is maintained for the time of the inparticular predominant, preferably complete, dissolution of magnesium inaluminum. The required time can be calculated by solving theone-dimensional diffusion equation knowing the diffusion constants ofmagnesium in aluminum. According to the invention, it may be useful tomaintain the temperature for a shorter or longer time.

The length of time for dissolving magnesium in aluminum is set inparticular at greater than 1 minute, preferably greater than 10 minutes,more preferably greater than 30 minutes, most preferably greater than 1hour, with utmost preference greater than 2 hours, and even morepreferably greater than 5 hours.

During the dissolution process, the pressure on the substrates that areto be bonded with one another is preferably maintained or increased. Theprevailing pressure is in particular greater than 1 Pa, preferablygreater than 100 Pa, more preferably greater than 10,000 Pa, mostpreferably greater than 1 MPa, with utmost preference greater than 10MPa, and even more preferably greater than 100 MPa. The forces used thatact on the wafer are greater than 10 N, preferably greater than 100 N,most preferably greater than 1,000 N, with utmost preference greaterthan 10,000 N, and even more preferably greater than 100,000 N.

During the dissolution process, the magnesium preferably dissolves inall of the aluminum. Because of the fact that the amount of magnesium tobe dissolved is very small and the amount of dissolving aluminum is verylarge, the total concentration of the magnesium in aluminum is verysmall. The magnesium is dissolved preferably not only exclusively in theregion of the aluminum that is near the surface but rather diffuses asdeep as possible into the aluminum, preferably so deep that after aspecific time, an even distribution of germanium in aluminum has beenachieved.

In a first procedure according to the invention, it is provided thatduring the cooling process, it does not result in exceeding the boundarysolubility of the magnesium in aluminum, so that magnesium alwaysremains completely dissolved in the aluminum. As a result, theprecipitation of a stoichiometric aluminum-magnesium phase in thealuminum matrix is prevented in the entire temperature range accordingto the invention. This is carried out according to the invention by aratio of aluminum layer thickness to magnesium layer thickness accordingto the invention being selected and by giving the diffusion processenough time until magnesium is distributed in the aluminum completely,and primarily over the entire available space. The ratio between themagnesium layer thickness and aluminum layer thickness is selected inparticular less than 1, preferably less than 10⁻³, more preferably lessthan 10⁻⁵, most preferably less than 10⁻⁷, with utmost preference lessthan 10⁻⁹, and even more preferably less than 10⁻¹¹.

In an alternative procedure according to the invention, the magnesiumlayer thickness is set in such a way that at higher temperatures, an inparticular predominant, preferably complete, dissolution of themagnesium is carried out, but during cooling, a supersaturated mixedcrystal is produced that leads to the precipitation of thestoichiometric aluminum-magnesium phase. The aluminum-magnesium phaseprecipitates can positively influence the strength properties of thealuminum. They preferably result in an increase in strength of thealuminum, in particular in combination with a heat treatment.

In quite special embodiments, the basic material is ground and/orpolished before the deposition of the protective material. In this case,it results in a planarization of the surface, which is of decisiveimportance for the later bonding process. The mean roughness and/or thequadratic roughness are less than 100 μm, preferably less than 10 μm,more preferably less than 1 μm, most preferably less than 100 nm, withutmost preference less than 10 nm, and even more preferably less than 1nm. The polishing can take place by purely mechanical and/or chemicalmeans. Most optimal is a chemical-mechanical polishing (CMP).

Should an oxide layer have formed on the basic material, the latter ispreferably already removed by the above-mentioned processes. Should theremoval of oxide by the above-mentioned processes not suffice, thealready mentioned processes for oxide removal, such as, for example,sputtering, use of reducing gases and/or acids, can be used in addition.After the planarization and optional purification of oxide, the coatingof the protective layer is then carried out.

For all embodiments according to the invention, it holds true accordingto an advantageous embodiment of the invention that the bonding processof a second heat treatment process that takes place later can take placein particular spatially separately. In the bonding unit, preferably onlythe actual bonding process is performed. The bonding process lasts inparticular less than 5 hours, preferably less than 1 hour, morepreferably less than 30 minutes, most preferably less than 15 minutes,and with utmost preference less than 5 minutes.

As soon as the bonding step was completed and a high enough adhesionbetween two substrates exists, the bonded substrate pair can be removedfrom the bonder in order to be further treated, in particularheat-treated, in another unit. Such a heat treatment unit is preferablya batch unit, therefore a unit that can simultaneously accommodate,perhaps even continuously, a large number of wafers. The heat treatmentin such a heat treatment unit is carried out according to the inventionin particular longer than 5 minutes, preferably longer than 30 minutes,more preferably longer than 1 hour, and most preferably longer than 5hours. The temperature in such a heat treatment unit is preferablyadjustable, preferably adjustable along a path and/or, as a function oftime, so that the processed substrates can run through exact temperatureprofiles. The temperatures that are used are in particular greater than25° C., preferably greater than 100° C., more preferably greater than300° C., more preferably greater than 500° C., and with utmostpreference greater than 800° C. The heat treatment can preferably takeplace in an inert gas atmosphere in order to protect the open surfacesof substrates against unnecessary or unintended oxidation.

In such a heat treatment plant, all conceivable heat treatment steps canbe performed. It would be conceivable in particular that the actualdissolution of the protective layer according to the invention into thebasic material is carried out only in the heat treatment unit. The heattreatment unit can bring several pairs of substrates at the same time toa higher temperature than would be the case in a bonding unit. In thecase of a continuously operating heat treatment unit that takes upsubstrate pairs on one side, continuously conveys them through the unit,for example by an assembly line, and releases them again on another end,even the setting of a temperature gradient over the path would beconceivable over time, and therefore, in particular, in the case ofconstant assembly line speed. According to the invention, the diffusionof the protective material into the basic material should only becarried out, however, outside of a bonding unit when no pressure isnecessary for the production of a corresponding solid bond between thesubstrates.

After making successful contact and a successful bond between thesubstrates, in particular after the dissolution of the protectivematerial into the basic material according to the invention, an attemptis made to achieve recrystallization of the structure, working as muchas possible over the entire thickness of the two basic material layers.This recrystallization can take place in the heat treatment unit if theprocess does not already take place during the actual bonding process.

The recrystallization leads to a new construction of the grains, inparticular via the bonding boundary surface, and thus produces amechanically stable, solid and permanent basic material layer thatextends through along the entire thickness. The new microstructureformed by recrystallization has the optimal and actually desiredstructure, since in this structure, a bonding boundary surface is nolonger present. Methods for at least partially controllablerecrystallization of the structure are preferably used. These include inparticular the increase in the dislocation density and/or acorrespondingly high temperature.

In the most preferred embodiment, the dissolution of the protectivematerial in the basic material according to the invention as well as therecrystallization of the structure are carried out in an external heattreatment unit, in particular separated from the bonder. As a result,the bonder is available for the next substrate bond as quickly aspossible. In a quite special embodiment, the dissolution processaccording to the invention as well as the recrystallization take placesimultaneously. Further advantages, features and details of theinvention are produced from the subsequent description of preferredembodiments and based on the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 a depiction of the binary Al—Ge phase diagram,

FIG. 2 a depiction of the binary Al—Ga phase diagram,

FIG. 3 a depiction of the binary Al—Zn phase diagram,

FIG. 4 a depiction of the binary Al—Mg phase diagram,

FIG. 5a a diagrammatic cross-sectional depiction of an embodiment of asubstrate according to the invention with a full-surface basic layerthat consists of a basic material and a full-surface protective layerthat consists of a protective material with alignment,

FIG. 5b a diagrammatic cross-sectional depiction according to FIG. 5awith a contact/bonding step, and

FIG. 5c a diagrammatic cross-sectional depiction according to FIG. 5aaccording to the bonding step.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a first binary Al—Ge system by way of example. Theimportant part in the phase diagram according to the invention is themixed crystal area 7. The mixed crystal area 7 is separated from thetwo-phase areas 9, 10 by the boundary solubility 8. The boundarysolubility for germanium decreases with decreasing temperature, startingfrom the eutectic temperature or the eutecticals 11. The boundarysolubility for germanium also decreases with increasing temperature,starting from the eutectic temperature or the eutecticals 11.

FIG. 2 shows a second binary Al—Ga system by way of example. Theimportant part according to the invention in the phase diagram is themixed crystal area 7. The mixed crystal area 7 is separated by theboundary solubility 8 from the two-phase areas 9, 10. The boundarysolubility for gallium decreases with decreasing temperature, startingfrom the eutectic temperature or the eutecticals 11. The boundarysolubility for gallium decreases also with increasing temperature,starting from the eutectic temperature or the eutecticals 11. Thedegradation of the eutectic by a eutectic point 6 that lies very near inthe concentration of the pure germanium is characteristic.

FIG. 3 shows a third binary Al—Zn system by way of example. Theimportant part according to the invention in the phase diagram is themixed crystal area 7. The mixed crystal area 7 is very pronounced here.At temperatures around 370° C., it reaches up to more than 65 mol % ofzinc. The mixed crystal area 7 is separated by the boundary solubility8′ from the two-phase area 10. The boundary solubility for zincdecreases with decreasing temperature, starting from the eutectoidtemperature or the eutectoids 11′.

FIG. 4 shows a fourth binary Al—Mg system by way of example. Theimportant part according to the invention in the phase diagram is themixed crystal area 7. The mixed crystal area 7 is separated by theboundary solubility 8 from the two-phase areas 9, 10. The boundarysolubility for magnesium decreases with decreasing temperature, startingfrom the eutectic temperature or the eutecticals 11. The boundarysolubility for magnesium also decreases with increasing temperature,starting from the eutectic temperature or the eutecticals 11.

FIG. 5a shows as simple a system according to the invention as possible,comprised of a first substrate 4 and a second substrate 5. Bothsubstrates 4 and 5 are coated with a basic material 1 and a protectivematerial 2. In an embodiment according to the invention, basic material1 and protective material 2 are not necessarily applied on the fullsurface on the first substrate 4 but rather have undergone a specificstructuring before the bonding. In this step, possible oxide layers ofthe protective material 2 have already been removed.

FIG. 5b shows a contact or bonding step of the two substrates 4 and 5.If the two substrates were structured, a previous orienting step wouldhave had to have oriented the two substrates to one another before theactual contact or bonding step would take place.

Finally, FIG. 5c shows the mixed crystal 12 that is produced and that iscarried out by the diffusion of the protective layer material 2 into thebasic material 1.

LIST OF REFERENCE SYMBOLS

-   1 Basic Material-   2 Protective Material-   3 Oxide Layer-   4 First Substrate-   5 Second Substrate-   6 Eutectic Point-   7 Mixed Crystal Area-   8 Boundary Solubility-   9 Two-Phase Area: Liquid, Solid-   10 Two-Phase Area: Solid, Solid-   11, 11′ Eutecticals, Eutectoids-   12 Mixed Crystal

Having described the invention, the following is claimed:
 1. A methodfor bonding a first substrate with a second substrate, said methodcomprising: applying an oxidizable basic material as a basic layer on abonding side of the first substrate, at least partially covering thebasic layer with a protective layer having a thickness of less than 100nm, the protective layer being comprised of a protective material, andbonding the first and second substrates, the bonding comprising at leastpartially dissolving the protective material in the basic material, thebonding being performed at a temperature that is less than a eutectictemperature of a eutectic system of the basic material and theprotective material.
 2. The method according to claim 1, wherein thebasic material is oxygen-affine and is comprised of aluminum and/orcopper.
 3. The method according to claim 1, wherein the step of applyingthe basic material as the basic layer and/or covering the basic layerwith the protective layer is/are carried out by deposition.
 4. Themethod according to claim 1, wherein the protective layer is appliedsuch that the basic layer is sealed at least predominantly relative tothe atmosphere.
 5. The method according to claim 1, wherein theprotective layer is treated before the step of bonding, said protectivelayer treated with one or more of the following processes: (a) chemicaloxide removal; (b) physical oxide removal, in particular with plasma;and (c) ion-assisted chemical etching.
 6. The method according to claim5, wherein said chemical oxide removal process includes a gaseousreducing agent and/or a liquid reducing agent.
 7. The method accordingto claim 5, wherein said ion-assisted chemical etching process includesfast ion bombardment, grinding and/or polishing.
 8. The method accordingto claim 1, wherein one or more of the following materials are selectedas the basic material and/or the protective material: (a) metals; (b)alkali metals; (c) alkaline-earth metals; (d) alloys; and (e)semiconductors, provided with corresponding doping.
 9. The methodaccording to claim 8, wherein said metals are selected from the groupconsisting of: Cu, Ag, Au, Al, Fe, Ni, Co, Pt, W, Cr, Pb, Ti, Te, Sn,Zn, and Ga.
 10. The method according to claim 8, wherein said alkalimetals are selected from the group consisting of: Li, Na, K, Rb, and Cs.11. The method according to claim 8, wherein said alkaline earth metalsare selected from the group consisting of: Mg, Ca, Sr, and Ba.
 12. Themethod according to claim 8, wherein said semiconductors are selectedfrom the group consisting of: element semiconductors and compoundconductors.
 13. The method according to claim 12, wherein said elementsemiconductors are selected from the group consisting of: Si, Ge, Se,Te, B, and Sn.
 14. The method according to claim 12, wherein saidcompound semiconductors are selected from the group consisting of: GaAs,GaN, InP, InxGal-xN, InSb, InAs, GaSb, AlN, InN, GaP, BeTe, ZnO,CuInGaSe₂, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, Hg(1-x)Cd(x)Te, BeSe, HgS,AlxGal-xAs, GaS, GaSe, GaTe, InS, InSe, InTe, CuInSe₂, CuInS₂, CuInGaS₂,SiC, and SiGe.
 15. The method according to claim 1, wherein the basicmaterial is Al and the protective material is selected from the groupconsisting of Ge, Ga, Zn, and Mg.
 16. The method according to claim 1,wherein the basic material is Al and the protective material is Ge. 17.The method according to claim 1, wherein the bonding of the first andsecond substrates further comprises bringing the bonding side of thefirst substrate into contact with the second substrate.
 18. The methodaccording to claim 1, wherein the bonding of the first and secondsubstrates comprises bringing the protective layer into contact with thesecond substrate.